In general, during machining, a wire electrode is transported between a pair of wire guides in a direction substantially vertical to XY plane while moving relatively to a work within XY plane. The travelling vertical wire electrode is a tool for electrical discharge machining. A machining fluid, such as water with high specific resistance, is supplied to a machining gap. In order to generate one electrical discharge for a predetermined duration (“ON time”), a voltage pulse is applied to the machining gap. A current flows through the machining gap due to the generation of an electrical discharge, and a small amount of material is removed from the work. The machining fluid washes away a machined chip and cools down the wire electrode.
When a predetermined OFF time is passed after the ON time is finished, the voltage pulse is applied to the machining gap again to generate the next electrical discharge. Thus, a series of current pulses is supplied to the machining gap. The ON time and the OFF time are very short and only a small amount of material is removed from the work by one electrical discharge. A wire electrical discharge machining device for machining the work like a coping saw is suitable to precision machining.
A typical wire electrode has a diameter of 0.1 mm to 0.3 mm. The wire electrode is usually transported to a pulling device via a plurality of pulleys and a pair of the wire guides from a wire bobbin. A pair of the wire guides is arranged above and below with respect to the work. The pulling device is controlled to maintain a traveling speed of the wire electrode at a set value. A tension applying device is located in a wire transportation path. The wire electrical discharge machining device controls the tension applying device and maintains a tension applied to the wire electrode at a set value. The tension of the wire electrode is, for example, set according to a diameter and a material of the wire electrode. If the tension is set to a large value, the wire electrode being substantially straight is ensured between the pair of the wire guides. The wire electrode being substantially straight contributes to high machining accuracy. However, an unduly large tension may cause undesirable breakage (wire breakage) of the wire electrode. Unduly large energy for electrical discharge machining is also easy to cause wire breakage.
The patent document 1 and the patent document 2 disclosed a wire electrical discharge machining device for surely maintaining a tension of a wire electrode at a set value.